diff --git a/src/libfetchers/registry.cc b/src/libfetchers/registry.cc
index 77d3b3378..f6760d2d0 100644
--- a/src/libfetchers/registry.cc
+++ b/src/libfetchers/registry.cc
@@ -155,7 +155,7 @@ void overrideRegistry(
 static std::shared_ptr<Registry> getGlobalRegistry(ref<Store> store)
 {
     static auto reg = [&]() {
-        auto path = settings.flakeRegistry;
+        auto path = settings.flakeRegistry.get();
 
         if (!hasPrefix(path, "/")) {
             auto storePath = downloadFile(store, path, "flake-registry.json", false).storePath;
diff --git a/src/libmain/common-args.cc b/src/libmain/common-args.cc
index f3f508ff4..51a61f1ca 100644
--- a/src/libmain/common-args.cc
+++ b/src/libmain/common-args.cc
@@ -31,19 +31,21 @@ MixCommonArgs::MixCommonArgs(const string & programName)
         .labels = {"name", "value"},
         .handler = {[](std::string name, std::string value) {
             try {
-                if (auto prefix = needsCompletion(name)) {
-                    std::map<std::string, Config::SettingInfo> settings;
-                    globalConfig.getSettings(settings);
-                    for (auto & s : settings)
-                        if (hasPrefix(s.first, *prefix))
-                            completions->insert(s.first);
-                }
                 globalConfig.set(name, value);
             } catch (UsageError & e) {
                 if (!completions)
                     warn(e.what());
             }
         }},
+        .completer = [](size_t index, std::string_view prefix) {
+            if (index == 0) {
+                std::map<std::string, Config::SettingInfo> settings;
+                globalConfig.getSettings(settings);
+                for (auto & s : settings)
+                    if (hasPrefix(s.first, prefix))
+                        completions->insert(s.first);
+            }
+        }
     });
 
     addFlag({
diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index 320b1b4b2..257b4b6c4 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -122,22 +122,15 @@ bool Args::processFlag(Strings::iterator & pos, Strings::iterator end)
     auto process = [&](const std::string & name, const Flag & flag) -> bool {
         ++pos;
         std::vector<std::string> args;
-        bool anyNeedsCompletion = false;
         for (size_t n = 0 ; n < flag.handler.arity; ++n) {
             if (pos == end) {
                 if (flag.handler.arity == ArityAny) break;
-                if (anyNeedsCompletion)
-                    args.push_back("");
-                else
-                    throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
-            } else {
-                if (needsCompletion(*pos)) {
-                    if (flag.completer)
-                        flag.completer(n, *pos);
-                    anyNeedsCompletion = true;
-                }
-                args.push_back(*pos++);
+                throw UsageError("flag '%s' requires %d argument(s)", name, flag.handler.arity);
             }
+            if (auto prefix = needsCompletion(*pos))
+                if (flag.completer)
+                    flag.completer(n, *prefix);
+            args.push_back(*pos++);
         }
         flag.handler.fun(std::move(args));
         return true;
@@ -209,14 +202,15 @@ Args::Flag Args::Flag::mkHashTypeFlag(std::string && longName, HashType * ht)
         .description = "hash algorithm ('md5', 'sha1', 'sha256', or 'sha512')",
         .labels = {"hash-algo"},
         .handler = {[ht](std::string s) {
-            if (auto prefix = needsCompletion(s))
-                for (auto & type : hashTypes)
-                    if (hasPrefix(type, *prefix))
-                        completions->insert(type);
             *ht = parseHashType(s);
             if (*ht == htUnknown)
                 throw UsageError("unknown hash type '%1%'", s);
-        }}
+        }},
+        .completer = [](size_t index, std::string_view prefix) {
+            for (auto & type : hashTypes)
+                if (hasPrefix(type, prefix))
+                    completions->insert(type);
+        }
     };
 }
 
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 4c8e2b26d..f2782ce69 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -1330,7 +1330,7 @@ bool statusOk(int status)
 }
 
 
-bool hasPrefix(const string & s, const string & prefix)
+bool hasPrefix(std::string_view s, std::string_view prefix)
 {
     return s.compare(0, prefix.size(), prefix) == 0;
 }
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 4be1d4580..a861d5aa6 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -431,7 +431,7 @@ template<class N> bool string2Float(const string & s, N & n)
 
 
 /* Return true iff `s' starts with `prefix'. */
-bool hasPrefix(const string & s, const string & prefix);
+bool hasPrefix(std::string_view s, std::string_view prefix);
 
 
 /* Return true iff `s' ends in `suffix'. */